J. Phys. Chem. 1995,99, 8851-8855
8851
Steric Considerations in the Amine-Induced Quenching of Luminescent Porous Silicon Robin R. Chandler-Henderson, Beata Sweryda-Krawiec, and Jeffery L. Coffer* Department of Chemistry, Texas Christian University, Fort Worth, Texas 76129 Received: February 7, 1995; In Final F o m : March 13, 1995@
In this work we report a study of the effects of the exposure of selected alkylamines on the visible photoluminescence (PL) of porous Si (PS), including their intrinsic quenching ability, time scales of restoration, and the additional influence of a weak acid (TFA) on the restoration of PL. While comparable percentages of porous Si PL can be quenched with a given amount of n-propyl-, n-butyl-, or n-pentylamine, monitoring the extent of PL restoration over time reveals discriminating behavior between n-pentylamine and n-propyl/ n-butylamine(s). As expected, increasing the porosity of the matrix does facilitate the restoration of PL. The introduction of trifluoroacetic acid (TFA) also speeds up the restoration of amine-quenched porous Si PL. Effects of organoamine exposure are also examined in light of any modification of the surface composition of porous Si, as probed by infrared spectroscopy (IR). The results reported here suggest that the diffusion of small amines within the porous Si matrix (as reflected in PL quenching/restoration behavior) is a function of the steric bulk of the amine chosen and the porous matrix where luminescent silicon nanoclusters reside.
Introduction Porous silicon (PS) is a material whose existence dates back to 1956, when Uhlir reported the electrochemical generation of a thin porous layer on top of crystalline silicon.’ Studies of this material focused primarily on its technological value in silicon-on-insulator (SOI) technology until 1990, when Canham reported that under certain preparative conditions electrochemical fabrication of such a layer on silicon created a strongly luminescent material.* This discovery has been a source of much excitement throughout the scientific community as chemists, physicists, and engineers have sought to understand the fundamental origins of the observed light emission in porous ~ilicon.~ It .has ~ also somewhat altered long-standing pessimism with regard to silicon’s possible exploitation in optoelectronic technology, a fundamental consequence of its indirect band gap and lack of efficient detectable l~minescence.~ The vast majority of these studies of light-emitting porous silicon have employed approaches common to physics and materials science. In contrast, chemical approaches to the study of porous silicon have been quite rare. Previous reports have specifically noted the effects of hydrocarbon solvent vapors,6 Bronsted acidshases,’ and Lewis bases8 on the photoluminescence (PL) of porous silicon. With particular regard to the effects of organoamine exposure, previous studies from our laboratories have shown that the addition of dilute heptane solutions of primary amines such as n-propylamine (C3H7NH2) strongly quenches the observed visible PL (maximum near 625 nm) of porous silicon fabricated from stain-etch methods.8 Bocarsly and co-workers have also noted the quenching of porous Si PL upon exposure of anodic-etch material to isopropylamine, diethylamine, diisopropylamine,and pyridine.7b The authors qualitatively noted in this report that such quenching was reversible upon exposure to trifluoroacetic acid (TFA), with the observed PL response attributed to a “proton-gated’ mechanism.7b In addition to assisting our further understanding of the mechanism of light emission, any fundamental studies of the chemical reactivity of the surface of porous silicon have
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absrracts, May 1, 1995.
important implications with regard to understanding the longterm stability of the porous silicon surface and its exploitation in chemical sensor applications. A priori, one anticipates that the chemical reactivity of the porous silicon network is a function of both chemical composition of the surface (Le. surface hydride versus oxide) and porosity, the distribution of pore sizes within the network structure. Recent studies by Lauerhaas and Sailor clearly demonstrate that the presence of silicon oxide groups rather than hydride at the porous Si surface manifests a radical modulation of the quenching behavior of the porous Si, particularly with regard to the effects of water vapor exposure? Thus, by maintaining a relatively fixed chemical surface composition, one can examine the influence of the porous silicon pore size and morphology on the suface chemical accessibility of luminescent centers in porous silicon. We report here the effects of the exposure of selected alkylamines on the visible photoluminescence of porous Si, including their intrinsic quenching ability, time scales of restoration, and the additional influence of a weak acid (TFA) on the restoration of PL. Effects of organoamine exposure are also examined in light of any modification of the surface composition of porous Si, probed by infrared spectroscopy (IR). The results reported here suggest that the ability of small amines to quench the PL of porous Si and subsequent restoration of light emission is a function of the steric bulk of the amine adsorbate. Experimental Section Porous Silicon preparation. Rectangular pieces (-5 mm x -30 mm) of p-type, (loo), Czochralski zone-grown (CZ-
grown), boron-doped Si (6-8 Q c m resistivity) were employed in a lateral anodic-etch process. The samples were etched galvanostatically at 3-4 mA/cm2 in a 5050 solution of 48%: 95% HF:ethanol for either (a) 5 min or (b) 30 min. After being etched, the wafers were rinsed with deionized water and dried in a stream of nitrogen. Gravimetric analyses of the resultant materials revealed void volumes of 65% and 86%, respectively.’O Titrant Solutions. Stock solutions of 0.01 M of each titrant (n-propylamine (98%), n-butylamine, n-pentylamine, trifluoroacetic acid [Mallinckrodt, Organic Reagents]); isopropylamine
0022-365419512099-8851$09.00/0 0 1995 American Chemical Society
Chandler-Henderson et al.
8852 J. Phys. Chem., Vol. 99, No. 21, 1995
(99%) [Aldrich]; and diisopropylamine [Matheson, Coleman and Bell] were prepared in heptane. All amines were distilled prior to use. Instrumentation. Steady-state PL measurements were recorded using a Spex Fluorolog-2 0.22-m double spectrometer at an excitation wavelength of 375 nm. All emission spectra were corrected for fluctuations in photomultiplier tube response. All Si samples were illuminated with the Xe lamp until steadystate PL intensity was reached. This steady-state process results in the photooxidation of the silicon surface, as evidenced by the growth of Si-0 stretching bands in the 1lOO-cm-' region of the infrared spectrum. Infrared spectra were recorded on a Midac Systems FTR with DTGS detector at a spectral resolution of 4 cm-'. Amine Titration Experiments. Photoluminescence measurements were performed on anodically-etched porous Si wafers by cutting the wafers into 5 x 5 mm pieces and placing them in a tapered Teflon holder with a small window at the end; porous Si samples were held secure there with a small nylon screw. The holder was designed to fit snugly into a 1-cm fluorescence cuvette and possesses a small (1-mm) orifice by which dilute heptane solutions of the organoamines can be added to or withdrawn. Great care was taken not to disturb the wafer position or to introduce additional oxygen into the cell at any time during a titration. Results reported are the average of six or more runs.
500
600
700
650
750
800
Wavelength (nm) Figure 1. PL spectra of anodically-etched porous Si, exposed to (i) 0.0 M, (ii) 1.3 x M, (iii) 1.3 x M, and (iv) 2.5 x M solutions of n-propylamine in heptane.
n-bntylamiae
0.8 0.6
0.2
Results and Discussion Anodized porous silicon is known to possess a convoluted array of Si wires and pores typically several microns thick, welldescribed by Lehmann and Gosele as a "quantum sponge"." Porous silicon derived from p-type substrates in particular exhibits quite small pore diameters and interpore spacings, typically between 1 and 5 nm.'* Given the porous nature of the matrix in which the luminescent silicon centers reside, an important question to address is the likelihood of a significant steric influence of an exposed Lewis base as reflected in its ability to quench the visible PL of porous Si. In order to evaluate such steric effects, a series of experiments comparing the quenching ability of n-propyl, n-butyl, and n-pentylamine were carried out for porous Si samples anodized from (100) p-type substrates; in each case, all samples studied possess relatively strong visible orange luminescence with maxima in the 610-620-nm region. Substantial reduction of the visible PL intensity of this material occurs upon addition of dilute solutions of amine. A gradual increase of amine concentration results in further quenching of PL until a steady-state level is reached (Figure 1). For the case of the primary amines studied, approximately 80-85% of the integrated porous silicon PL can be quenched, irrespective of the nature of the amine. However, one can discriminate between amines somewhat by examining the time scale of restoration of the porous silicon PL. This is illustrated for the PL of anodized porous Si of approximately 65% porosity and exposed to the three types of amines in Figure 2. In this series of experiments, after the amine-exposed PS has reached steady-state PL intensity, the amine solution is removed and a fresh solution of pure heptane is introduced to the cell followed by vigorous stirring. Three general observations are noted: (1) the restoration of PL in these samples is a sluggish process, requiring '2 h to restore approximately 85% of the original PL intensity for n-butylamine, for example; (2) both n-propyl- and n-butylamine exhibit similar behavior in terms of the time scale of PL restoration; and (3) recovery from n-pentylamine exposure is markedly slower, with only -40% of the integrated PL intensity restored in 1 h.
550
4 I
0
I
1
10
20
I
I
I
I
1
30
40
50
60
70
Time (min) Figure 2. Restoration of the PL of anodically-etched porous Si as a M solutions of function of time after quenching with 3 x n-propylamine, n-butylamine, and n-pentylamine in heptane. Immediately prior to t = 0, the amine solution was removed and a fresh aliquot of pure heptane was introduced, followed by vigorous stirring. 1 0
0.8
n-pentylamine / 65% porosity PS n-pentylamine / 86% porosity PS
0.6 0
s
0.4
0.2
0
I I
I
0
20
I
40 60 Time (min)
80
Figure 3. Effect of increasing the porosity of the silicon matrix (65% versus 86% porosity) on the restoration of PL of porous silicon after quenching with a n-pentylamine solution (3 x M) in heptane and followed by a heptane wash as described above.
Apparently, the conformational flexibility of this five carbon chain is enough to impede diffusion of these molecules in a geometrically-restricted porous environment. This appreciable difference in the behavior of n-pentylamine led us to next examine the effect of increasing the porosity of the silicon matrix for this same amine on the restoration process (Figure 3). It is found that increasing the porosity of the porous layer to approximately 86% results in a substantial increase in the amount of PL which can be recovered per unit time; Le. now approximately 60% of the original PL can be restored in
Amine-Induced Quenching of Luminescent Silicon I
I
- photooxidized PS $ 1.2-
3
----+ -.I-- +
J. Phys. Chem., Vol. 99, No. 21, 1995 8853 I
2.5 I
I
-
5 min in propylamine 30 min in propylamine
'
I
I
I
I
I
+ TFA
I
3 x 10m4
es:
p 1.1-
c d Y
1I
I
I
1200
900
600
Wavenumber (cm") Figure 4. Infrared spectra of a typical anodized porous silicon substrate employed in this work in the region of 500-1400 cm-': freshly-
prepared sample after UV illumination to steady-state photoluminescence intensity (solid line); the same wafer after 5-min exposure to a solution of a n-propylamine (3 x M) in heptane (dashed line); the same wafer after 30-min exposure to a solution of a n-propylamine M) in heptane (mixed soliddashed line). (3 x
60 min. This strongly suggests that the structure of the stabilizing matrix itself also impacts on how these organoamines influence the PL of porous Si. In light of these observations, an extremely important issue to address is what happens to the surface composition of the porous Si during amine exposure. This is convienently monitored by an examination of changes in the IR spectra of the porous Si before/after amine exposure. These experiments are carried out in a variable path length IR cell modified to permit solvent removal and handling in an inert atmosphere. Relevant results are illustrated in Figure 4. Exposure of the porous Si surface to a solution of n-propylamine (3 x M) in heptane for a period of 5 min followed by removal of the solution in vacuo results in minor oxidation of the porous Si surface, as evidenced by a slight (12 f 2%) increase in the intensity of the Si-0-Si band near 1100 cm-' (Figure 4, dashed line) compared to the as-prepared PS sample. The intensity of the v(SiH,) modes near 2100 cm-' do not experience significant changes in intensity during this time frame. However, with increasing duration of amine exposure, the porous Si surface continues to oxidize; after 30 min, for example, the silicon oxide mode near 1100 cm-' has increased in intensity by roughly 50 & 11%. This increased oxidation of the porous Si surface induced apparently by the amine suggests, in part, why the porous Si does not seem to recover all of the original PL intensity after prolonged periods of exposure (>30 min)! Another possible concomitant process to note here is that some subtle amine induced changes in local surface morphology of the porous Si layer may also occur, thereby inducing some irreversible behavior as well. While the details of such a mechanism are not yet available, it is assumed that a nucleophilic process similar to the known halide-induced oxidation of porous Si is operative in this case.g In this case, the oxygen is already present in the pores, being acquired from rapid uptake into the porous Si matrix upon air exposure after fabrication. Effects of Acid Exposure after Amine Quenching. Given the relatively sluggish restoration of the porous Si luminescence after amine exposure, it was then decided to examine the impact of a weak acid in order to facilitate the restoration of photoluminescence of these quenched surfaces. We chose trifluoroacetic acid, since Bocarsly and co-workers have shown that TFA by itself does not quench porous Si PL and it can be used to restore the PL of porous silicon which was quenched by exposure to isopropylamine, diethylamine,. diisopropylamine,
0.5
I
I
I
1
1
I
40 60 80 Time (min) Figure 5. Effect of exposure of the porous silicon substrate to a dilute solution ( M) of TFA and monodeuterated TFA on the integrated PL of porous silicon as a function of time. 0
20
and ~ y r i d i n e . ~We thus examined how TFA affects the restoration of porous Si quenched by an amine with regard to (a) a short term time scale ('3 min), and (b) its time-based response. Because the data were not available, we first examined the time-based response of porous Si to TFA itself. Interestingly, for our porous Si substrates, exposure to dilute solutions M) of TFA actually enhances the steady-state PL of porous Si. The magnitude of the enhancement varies appreciably from sample to sample (see Figure 5), but for short exposure times (< 3 min) the enhancement ranges from 25 to 50%. The magnitude of this enhancement diminishes slightly with time but still remains higher than the non-TFA-treated sample after > 1-h exposure. Although the precise mechanism for this enhancement is not clear at this time, TFA presumably acts to passivate surface defects acting as nonradiative centers.7b Comparable experiments with deuterated TFA (CSCOOD) induced an even larger overall increase in integrated porous Si PL intensity (Figure 5 ) , perhaps suggesting that energy transfer via bridging -OH(D) linkages are important to this process. Infrared spectroscopic analysis of the porous Si surface composition before and after TFA exposure does not reveal any significant changes in either silicon hydride or silicon oxide vibrational modes. Having determined the effect of TFA exposure alone on the PL of porous Si, the question of how TFA can alter the luminescence of porous Si already quenched by organoamines was then addressed. In the first series of experiments, the porous Si PL was quenched by exposure to a given 3 x M amine solution and the PL recorded within 3 min. This was immediately followed by exposure to 6 x M of TFA with the corresponding PL spectrum also recorded within 3 min of acid addition. Under these conditions, the extent of PL restoration upon TFA addition is found to be somewhat dependent upon the steric volume of the amine quenching the porous Si PL (Table 1). For example, after addition of 3 x M of TFA, it is found M amine followed by 6 x that approximately 60% of the original integrated PL remains for n-propylamine, 33% for n-butylamine, and 20% of npentylamine; in fact, addition of TFA to the n-pentylaminequenched sample results in further quenching on a 3-min time scale. Other amines reveal similar trends in terms of restoration of porous Si photoluminescence after TFA addition. A comparison of isopropylamine and diisopropylamine exposure onto anodically-etched porous Si demonstrated that the PL of the sample quenched by the secondary amine could not be restored on this time scale with "FA addition, while approximately 45% of the original PL can be restored with TFA to a sample whose PL was quenched by isopropylamine.
Chandler-Henderson et al.
8854 J. Phys. Chem., Vol. 99, No. 21, 1995 TABLE 1: Reversibility of Amine Adsorption onto Anodically-EtchedPorous Silicon Wafers upon Addition of TrifluoroaceticAcid (TFA)
.-3
% PL intens remaining
I
c)
after quenching concn (M) with amines
amine n-propylamine n-butylamine n-pentylamine isopropylamine diisopropylamine
3 3 3 5 5
x x x x
I
.-nn E
60( 10) 33(6) 20(6) 45(12) lO(10)
21(3) 24(5) 22(5) 10(2) 12(10)
x
C
after addition of 2 equiv of F A n
w 500
A 2-equiv amount of TFA = 2 x (total equiv of amine added to the system). Associated standard deviations for each category are listed in parentheses. a
2.5
,
PS I n-propylamine I TFA I
I
o A
0
0
1:l.S k1.25 1:l
20
I
I
I
Amine / TFA Amine I TFA Amine I TFA
40
Time (min)
60
c
80
Figure 6. Changes in the integrated porous Si PL as a function of time after exposure to a 3 x n-propylamine solution in heptane followed by varying amounts of TFA. Thus, from this data there is some indication of a modest steric effect. However, given the response of the primary amines with regard to porous Si luminescence as a function of time (described above), the resultant standard deviations in these short-term measurements infers that the system may not have reached equilibrium in terms of optimal diffusion of amine to the surface and resultant quenching. Therefore, the time-based response of a typical porous Si/amine/TFA system was then evaluated. The results are illustrated in Figure 6 for the case of n-propylamine quenched PL of porous Si, followed by varying amounts of TFA addition (time zero here is with respect to the point of acid addition). Several points are to be noted from this graph. First, the extent of the restoration of PS PL within the first 2-3 min increases as the amount of TFA added increases. Second, there apparently is little appreciable difference in the time-based response of n-propylamine by itself and n-propylamine and TFA in a 1:l ratio. If anything, the presence of the TFA actually slows down the intrinsic process slightly, perhaps as a consequence of crowding the pores. Yet, with excess amounts of TFA, it would appear that, given enough time, the ability of TFA to enhance porous Si PL can be observed as the PL integrated intensity rises > 100% in the case of 1:2 amine/TFA and 1:1.5 amine/TFA. On the basis of these observations, it seems reasonable to suggest that the amine kinetically blocks the porous Si surface from TFA and thereby slows down the intrinsic ability of TFA to enhance the PL. However, it cannot be absolutely discemed at present whether these effects arise from complex acid-base chemistry in the solution within the pores of the material or rather a facilitated desorption of some type of PS-amine surface adducts. As noted earlier in the PL restoration experiments solely with organoamines, the intrinsically sluggish response of the porous Si samples whose PL was quenched by n-pentylamine could
550
600
650
700
750
800
Wavelength (nm) Figure 7. Reversible addition of n-pentylamine to an anodic-etch wafer which was subjected to an open circuit etch for 2 h: (i) initial spectrum; M; (iii) [TFA] = 1.2 x (ii) [n-pentylamine] = 2.5 x
M.
+
+
be altered by increasing the pore size. An increase in pore size should also then affect any TFA-facilitated restoration of PS PL already quenched by amine exposure, primarily in terms of the magnitude of PL restored in a given time interval. Subsequently, a porous Si sample was prepared in which the normal 30-min anodic etch was followed by a 120-min “open circuit” etch (electrodes disconnected but the PS sample remains in the electrolyte). This sample, prepared by a conventional 30-min anodic etch followed by a 2-h open circuit etch, was exposed to n-pentylamine (an amine which showed no shortterm resoration of PL on wafers etched anodically for 30 min). Subsequent exposure to TFA revealed that n-pentylamine adsorption was partially restored (ca. 23%) within 3 min on the 2-h open circuit-etched wafer, albeit only with a large excess of acid (Figure 7). As noted previously, the postfabrication open circuit etch gives rise to larger pores with a corresponding smaller total surface area within the porous l a ~ e r . ~ .This ’~ conclusion is supported by differences in the relative surface areas of these two types of anodic-etch materials. Lowtemperature BET gas adsorption measurements (employing Kr) reveal a 25% decrease in surface area for the standard anodicetch sample upon a 120-min open circuit etch in ethanolic HF (605 m2/g versus 456 m2/g). Thus, even in a more complex temary system (amine/TFA/porous Si), the alteration of pore structure can impact the restoration of porous Si PL. Conclusions In general, these results suggest that preparative conditions may be manipulated to induce some selectivity in terms of exposure of small molecules to the surface of porous silicon, a point which may prove useful in possible sensor applications. This is particularly illustrated in this work with respect to differences in the time scale of luminescence restoration between n-propyl-, n-butyl-, and n-pentylamine. Overall, it is important to point out that the restoration of porous silicon photoluminescence is a sluggish, complex phenomenon with regard to exposure to strong Lewis bases such as organoamines, and it is important that the porous Si preparative conditions employed for a given sample be considered when evaluating andor comparing results of this type. Acknowledgment. We gratefully thank the Donors of the Petroleum Research Fund, administered by the American Chemical Society; the Texas Advanced Technology Program; and the Robert A. Welch Foundation for their financial support of this research. Support by Texas Instruments, Inc., in the form of a generous supply of silicon wafers is also gratefully acknowledged. We also thank David Smith of Micromeritics
Amine-Induced Quenching of Luminescent Silicon for providing the low-temperature Kr surface area measurements and the reviewers for their helpful comments.
References and Notes (1) Uhlir, A. Bell Syst. Tech. J. 1956, 35, 333. (2) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (3) For example, see the volumes: (a) Light Emission from Porous Silicon; Iyer, S., Collins, R., Canham, L. Eds.; Materials Research Society Sympsium Proceedings 256; Materials Research Society: New York, 1992. (b) Microcrystalline Semiconductors: Materials Science and Devices;
Fauchet, P., Tsai, C. C., Canham, L., Shimizu, I., Aoyagi, Y., Eds.; Materials Research Society Symposium F’roceedings 283; Materials Research Society: New York, 1993. (4) (a) Brus, L. Adv. Muter. 1993, 5, 286. (b) Kavanagh, K.; Sailor, M. J. Adv. Muter. 1992,4, 432. Vial, J. C., Billat, S.; Bsiesy, A.; Fishman, G.; Gaspard, F.; Herino, R.; Ligeon, M.; Madeore, F.; Mihalcescu, I.; Muller, F.; Romestain, R. Physica B 1993, 185, 593.
J. Phys. Chem., Vol. 99, No. 21, 1995 8855 ( 5 ) Iyer, S. S.; Xie, Y . H. Science 1993, 260, 40. (6) Lauerhaas, J. M.; Credo, G.; Heinrich, J.; Sailor, M. J. J. Am. Chem. SOC. 1992, 114, 1911. (7) (a) McCord, P.; Yau, S.-L.; Bard, A. J. Science 1992, 257, 682. (b) Chun, J.; Bocarsly, A.; Cottrell, T.; Benziger, J.; Lee, J. J. Am. Chem. SOC.1993, 115, 3024. (8) Coffer, J. L.; Lilley, S. C.; Martin, R. A.; Files-Sesler, L. J. Appl. Phys. 1993, 74, 2094. (9) Lauerhaas, J. M.; Sailor, M. J. Science 1993, 261, 1567. (10) Brumhead, D.; Canham, L.; Seekings, D.; Tufton, P.Electrochim. Acta 1993, 38, 191. (11) Lehmann, V.; Gosele, U. Adv. Mater. 1992, 4 , 114. (12) Smith, R. L.; Collins, S. D. J. Appl. Phys. 1992, 71, R1. (13) Robinson, M. B.; Dillon, A. C.; George, S. M. Appl. Phys. Lett. 1993, 62, 1493.
JP950400M
